The present process and apparatus relate to processes for the impregnation of porous parts. In particular, the present process and apparatus provide for control of the extent of impregnation by measuring the change in buoyancy of the parts during the impregnation process.
Impregnation of porous parts is a common technique employed in a variety of industries for a variety of reasons. Stone, brick, ceramic, wood, polymer, aggregate, cermet, and porous metal parts, for example, are commonly impregnated. Typically, a sealant is impregnated into the part because the porosity is undesirable in the intended end use of the part. In some applications, it is only necessary to seal the pores on the surface of the part. In other applications, thorough impregnation of the part is necessary. Further, in certain applications it may be possible to over-impregnate a part, so careful control of the level of impregnation is required.
By way of example, fuel cells, including solid polymer electrolyte fuel cells, utilize initially porous components such as separator plates. Separator plates are commonly made from graphite, graphitized carbon or carbon-resin composites.
Separator plates are typically thoroughly impregnated with an impregnant that assists in imparting necessary impermeability and mechanical stability (that is, structural strength and hardness). In other words, once impregnated, separator plates are substantially impermeable to the fluid reactants and/or coolants used in the fuel cell or fuel cell stack, mechanically stable and electrically conductive. Known impregnants suitable for such purposes include phenols, epoxies, melamines, furans, and acrylics, such as methacrylates, for example.
For example, expanded graphite sheets, such as the material available from UCAR Carbon Technology Corp. (Danbury, Conn., U.S.A.) under the tradename GRAFOIL, may be used to form separator plates for fuel cells. Expanded graphite sheets are useful in this regard because they are relatively light, flexible and amenable to low-cost manufacturing methods, such as embossing. Nonetheless, separator plates made from expanded graphite sheets are typically impregnated in order to achieve the desired levels of impermeability and mechanical stability.
It is important that such plates be sufficiently impregnated to meet performance requirements. At the same time, it is possible to over-impregnate the plates, resulting in degradation or loss of desired structural and/or functional properties.
In addition, it is generally undesirable to have residual cured impregnant left on the surface of the impregnated plates. The presence of impregnant deposits on the surface of the cured plate can: affect the electrical conductivity of the plate; interfere with electrical contact between fuel cell components in the assembled cell/stack; be detrimental insofar as thickness tolerances are concerned; and, may also interfere with the function of surface features on the plate. Accordingly, impregnation process control is an important aspect of separator plate manufacture.
In typical industrial processes, curing of the impregnated parts is accomplished by dipping the parts in a hot water bath after washing and rinsing. Often, the washing, rinsing and curing steps can occur in the same vessel.
Conventional impregnation process control methods typically rely on a consistent time required to sufficiently impregnate a part. Based on such methods, an optimum time can be selected to ensure adequate impregnation without much wasted time or expenditure. However, where relatively subtle process and/or material changes can drastically affect the proper impregnation time necessary to achieve the desired impregnation level, such methods are unsatisfactory. For example, the variability of different grades, lots and batches of expanded graphite sheet, as well as variations in separator plate processing or design, has made it virtually impossible to determine an appropriate impregnation time beforehand for a given lot of separator plates.
Current methods use the impregnation time from the previous batch of plates as the initial time estimate for impregnation of the next batch, taking into account other factors such as plate thickness, density, etc. Since the level of impregnation can only be assessed after the impregnation process is complete, entire batches of parts may have to be scrapped due to incorrect estimates of the impregnation time. This approach is costly in terms of time and materials, and is poorly suited to high-volume production methods.
In one embodiment, the present process comprises:
(a) immersing at least one porous part in an impregnant;
(b) measuring at least one parameter indicative of the buoyancy of the part(s) as the impregnant impregnates the part(s); and
(c) interrupting impregnation when the measured parameter(s) indicates a predetermined level of impregnation is achieved.
The measured parameter may comprise the change in weight of the part(s), the rate of change in weight of the part(s), or both. Preferably, the measured parameter(s) is (are) measured continuously. Impregnation may be interrupted when the change in weight exceeds a predetermined threshold value, when the rate of change in weight falls below a predetermined threshold value, or both. The measured parameter(s) may be compared to a reference parameter value and impregnation may be interrupted when the measured parameter(s) varies from the reference parameter value(s) by less than a predetermined threshold amount. For example, impregnation may be interrupted when the measured parameter indicates that at least 85% of the void volume of the porous part(s) is impregnated, or alternatively, when the measured parameter indicates that at least 95% of the void volume of the porous part(s) is impregnated.
The process may further comprise sending an output signal representative of the measured parameter(s) to a controller, which may comprise a display for displaying the measured parameter(s) represented by the output signal(s). Impregnation may be interrupted in response to an output signal from the controller.
The porous part may comprise a carbon plate, including but not limited to a graphite plate. For example, the porous part may comprise an expanded graphite plate.
The impregnant may be any suitable impregnant. Where graphite plates are impregnated, particularly suitable impregnants include resins such as phenols, epoxies, melamines, furans and acrylics such as methacrylates, for example.
The porous part(s) may be impregnated at any suitable pressure. For example, the porous part(s) may be impregnated at ambient pressure, at a pressure less than atmospheric pressure, at a pressure greater than atmospheric pressure, or a combination thereof.
Where a plurality of porous parts is impregnated according to the present method and apparatus, the measured parameter may be indicative of the level of impregnation of all of the porous parts or only a portion thereof. For example, the measured parameter may comprise the change in weight, rate of change in weight, or both, of all of the porous parts being impregnated. Alternatively, the measured parameter may comprise the change in weight, rate of change in weight, or both, of a representative sample of the porous parts being impregnated.
The present process may further comprise heating the porous part(s) before immersion into the impregnant.
The present process may also further comprise washing, rinsing and drying the impregnated part(s) to remove at least a portion of residual water that may be present on the surfaces thereof.
In another embodiment, the present process comprises preparing an impregnated porous part for curing by washing and rinsing the impregnated porous part, and drying the impregnated part at a drying temperature to remove at least a portion of the residual water from the surfaces of the part.
An apparatus for impregnating porous parts is also provided comprising a vessel for holding at least one porous part and an impregnant, and at least one measuring device for measuring the change in weight of the part(s) immersed in the impregnant within the vessel. The apparatus may further comprise a pump fluidly connected to the vessel for varying the pressure therein from ambient pressure.
The measuring device(s) may comprise an electronic balance having a cantilever arm connected at one end to the balance, the other end of the arm being suspended in the vessel. The suspended end being removably attachable to the porous part(s) for measuring the change in weight thereof. Alternatively, the measuring device(s) may comprise a load cell associated with the interior of the vessel. Preferably, the at least one measuring device generates output signals representative of the measured parameter(s). The apparatus may further comprise a controller for receiving the output signals from the measuring device(s), and the controller may comprise a display for displaying the change in weight represented by the output signals.